This invention relates generally to integrated circuits, and more particularly to Micro Electro-Mechanical System (MEMS) devices.
In the telecommunications industry, the demand for lightweight portable devices such as personal computing devices, Personal Digital Assistants (PDA""s) and cellular phones has driven designers to reduce the size of existing components. A Q value is a ratio of the power stored in a device to the dissipated power in a device. Due to the need for Q values beyond the capabilities of conventional IC technologies, board-level passive components continue to occupy a substantial portion of the overall area in transceivers of handheld telecommunications equipment, presenting a bottleneck against further miniaturization. For example, discrete components currently occupy approximately 50% of the space in cellular phones.
Recently MEMS devices including resonators, filters, and switches have been developed that offer an alternative set of strategies for transceiver miniaturization and improvement. MEMS devices are high-Q, chip-level, lower power replacements for board-level components that greatly decrease space and area requirements.
One such MEMS device is an RF switch for switching RF signals, shown in a cross-sectional view in FIG. 1. RF drumhead capacitive MEMS switch 10, disclosed by Goldsmith et al. in U.S. Pat. No. 5,619,061, comprises an insulator 14 such as SiO2 deposited over a substrate 12, which may comprise silicon, for example. A bottom electrode 16 is formed on insulator 14 and a dielectric 18 is formed over bottom electrode 16. Capacitor dielectric 18 typically comprises Si3N4, Ta2O5 or other suitable dielectric materials, for example. An active element comprising a thin metallic membrane 22 is suspended away from electrode 16 by an insulating spacer 20. Membrane 22 which serves as a top electrode is movable through the application of a DC electrostatic field between membrane 22 and bottom electrode 16. Membrane 22, dielectric 18 and bottom electrode 16 comprise a metal-dielectric-metal capacitor when the MEMS switch 10 is in the xe2x80x9conxe2x80x9d position, shown in FIG. 2. In the xe2x80x9coffxe2x80x9d position shown in FIG. 1, with no voltage applied to membrane 22 and bottom electrode 16, the capacitance value is at a minimum. MEMS switches 10 have low insertion loss, good isolation, high power handling, and very low switching and static power requirements.
A MEMS switch 10 may be designed for use as a varactor. A varactor is a discrete electronic component, usually comprising a P-N junction semiconductor, designed for microwave frequencies, in which the capacitance varies with the applied voltage. Varactors are sometimes referred to as tunable capacitors. Varactors are used in frequency up and down conversion in cellular phone communication, for example. Existing varactors are usually p-n diodes specifically designed for operation in the reverse bias regimes where the capacitance(CJ) of the depletion region is varied to set frequency (xcfx890) of operation as reflected in Equation 1:
xcfx890≈1/(CJ*RS*RP)xc2xdxe2x80x83xe2x80x83Equation 1:
where resistances RP and RS are the parallel and series resistances of the diode, respectively. Some primary requirements of a varactor are that it have a high quality factor (Q) for increased stability to thermal variations and noise spikes, and a large linear tuning range (TR). High-performing varactors are usually made of GaAs. Unfortunately, these devices use a different processing technology that is not amenable to integration into standard Si-CMOS process.
MEMS devices offer a means by which high Q large tuning range varactors can be integrated in higher level devices such as voltage controlled oscillators and synthesizers using the current Si-CMOS process. The drumhead capacitive switch 10 shown in FIG. 1 may be designed to produce a MEMS varactor. The voltage across the electrodes is varied to pull down and up membrane 22, which varies the distance Dair between membrane 22 and dielectric 18, which changes the capacitance of the device 10 accordingly.
A problem in MEMS devices is stiction, which is the unintentional adhesion of MEMS device 10 surfaces. Stiction may arise from the strong interfacial adhesion present between contacting crystalline microstructure surfaces. The term stiction also has evolved to often include sticking problems such as contamination, friction driven adhesion, humidity driven capillary forces on oxide surface, and processing errors. Stiction is particularly a problem in current designs of MEMS varactors, due to the membrane 22 possibly adhering to dielectric 18, resulting in device 10 failure, either temporarily or permanently. To prevent stiction, material and physical parameters, and voltage signal levels of the varactor are designed to avoid contact of membrane 22 with dielectric 18. Coatings such as Teflon-like materials that resist stiction are frequently applied over dielectric 18.
The present invention achieves technical advantages as a MEMS varactor designed to operate in a stiction mode. The pull-down electrode or top membrane maintains contact with the underlying dielectric covering the bottom electrode during operation of the varactor. As the voltage across the pull-down electrode and the bottom electrode is varied, the area of the pull-down electrode contacting the dielectric is varied, which varies the capacitance.
Disclosed is a MEMS varactor, comprising a bottom electrode formed over a substrate, a dielectric material disposed over the bottom electrode, and a spacer proximate the bottom electrode. A pull-down electrode is disposed over the spacer and the dielectric material, wherein the varactor is adapted to operate in a stiction mode.
Also disclosed is a method of manufacturing a MEMS varactor, comprising depositing an insulator on a substrate, forming a bottom electrode on the insulator, and depositing a dielectric material over the bottom electrode. A spacer is formed over the insulator, and a pull-down electrode is formed over the spacer and the dielectric material, wherein the varactor is adapted to operate in a stiction mode.
Further disclosed is a method of operating a MEMS varactor, comprising applying a voltage across the bottom electrode and the pull-down electrode to produce a predetermined capacitance across the bottom and pull-down electrode, wherein at least a portion of the pull-down electrode is adapted to contact the dielectric material during operation in a stiction mode.
Advantages of the invention include solving the stiction problems of the prior art by providing a varactor adapted to operate in a stiction mode. The present MEMS varactor is a high Q varactor having a large tuning range. The distance between the dielectric and the membrane may be increased in accordance with the present invention, allowing for a larger tuning range and providing more sensitivity to a change in voltage. A wider range of voltages and capacitances is available with the present MEMS varactor design. Furthermore, the use of Teflon-like coatings on dielectric to prevent stiction of membrane is not required, as in some prior art designs. A wider variety of dielectric materials may be used for dielectric than in the prior art because there is no need for concern about stiction of the membrane to the dielectric. The invention provides an extended tuning range that is not possible with only an air gap for the capacitive medium.